1. Field of the Invention
The present invention relates to a battery residual amount display circuit and particularly, to a battery residual amount display circuit for displaying the residual amount of a battery used in an apparatus having a plurality of operation modes, such as a radio apparatus.
2. Description of the Prior Art
The Li (lithium)-ion battery has a feature that the battery voltage decreases approximately like a first-order function as it discharges. Therefore, it is possible to know the residual amount of a battery by measuring its output voltage. In an apparatus using a Li-ion battery as a power supply, the residual amount of the battery is determined by measuring its output voltage and displayed on an indicator.
For example, Japanese Unexamined Patent Publication No. Hei. 6-224844 discloses a radio apparatus having a battery residual amount display circuit. In this radio apparatus, as shown in FIG. 7, the output voltage of Li-ion battery 71 is converted into an operation voltage by power supply circuit 72, to thereby produce a stable power supply voltage. On the other hand, the output voltage of Li-ion battery 71 is converted by A/D converter 73 into a digital value, which is detected by control section 74. The battery residual amount is judged based on a detected voltage value, and a judged battery residual amount is displayed on indicator 75.
Having, for instance, a microprocessor as a main control unit, control section 74 controls a radio circuit section (not shown). At the same time, control section 74, specifically its residual amount judging section 76, compares the battery output voltage with a plurality of threshold values and thereby judges which of a plurality of levels the battery residual amount belongs to. In performing the above judgment, to eliminate a measurement error depending on the operation mode, the residual amount judging section 76 changes the threshold values based on mode information that is supplied from the main control unit. The operation of residual amount judging section 76 will be described below with reference to FIG. 8.
In residual amount judging section 76, first, at step S81, the operation mode of the radio apparatus is checked. If the operation mode is a reception-waiting mode, at step S82 the process waits for a time slot other than a transmission slot. The battery voltage is measured at step S83. Then, at step S84, the battery residual amount is judged by comparing the measured battery voltage with threshold values that were stored in advance.
If the operation mode is judged to be a talking mode at step S81, residual amount judging section 76 operates in the same manner as in the case of the reception-waiting mode. That is, the process waits for a time slot other than a transmission slot at step S85, and the battery voltage is measured at step S86. The battery residual amount is judged at step S87 by comparing the measured battery voltage with threshold values. However, the threshold values used in the talking mode is different from those used in the reception-waiting mode. This is because the discharge current (power consumption) in the talking mode is larger than in the reception waiting mode and the battery voltage is lowered accordingly.
The battery residual amount that has been measured in the above manner is displayed on indicator 75 at step S78.
In the conventional battery residual amount display circuit, the battery residual amount is judged based on the battery voltage on the assumptions that the discharge current (power consumption) is constant in each mode and that the relationship between the discharge current and the voltage drop never changes. However, the discharge current varies even in the same mode and the relationship between the discharge current and the voltage drop varies with the ambient temperature and the number of charge/discharge cycles. Therefore, the conventional battery residual amount display circuit has a problem that the battery residual amount cannot be judged in a stable manner.
Moreover, the residual amount of a battery is not judged in talking mode so as to avoid the influence of voltage drop in transmission slot, thus there was a disadvantage that the battery residual amount judgement becomes inaccurate in the apparatus which has lengthy talking mode.